[Study on nocturnal penile tumescence (NPT) in healthy males. Study on the relationship between the serum free testosterone level and NPT].

We studied the correlation of nocturnal penile tumescence (NPT) and serum free testosterone, which were evaluated in 123 healthy males and in 6 cases of primary hypogonadotropic hypogonadism before and after treatment with human chorionic gonadotropin (hCG). In healthy subjects 3 to 84 years old, the serum free testosterone levels had a relationship with 4 parameters revealed by NPT: total tumescence time, mean tumescence time, maximum penile circumference change and maximum increase rate in penile circumference. In men who showed the peak value for each NPT parameter and those who were younger, the serum free testosterone levels were correlated with all 6 parameters revealed by NPT: total tumescence time, NPT frequency, mean tumescence time, circumference in flaccid penis, maximum penile circumference change and maximum increase rate in penile circumference. In 6 cases of the primary hypogonadotropic hypogonadism, total tumescence time, mean tumescence time, maximum penile circumference change, and maximum increase rate in penile circumference significantly increased after hCG treatment. This indicated that an increase of the serum free testosterone level was responsible for the increase of the parameters of NPT. In men who were older than the subjects showing the peak value for each NPT parameter, the serum free testosterone levels were correlated with 4 parameters revealed by NPT: total tumescence time, mean tumescence time, maximum penile circumference change and maximum increase rate in penile circumference. These results suggest that the free testosterone level may be involved in the decrease of maximum penile circumference change as well as in age-related changes of the penile vascular system that were the major factor responsible for the decrease. This is the first report that studied the relationship between NPT and androgen in any group. NPT as a parameter for the erectile function was related to androgen levels.